1. Field of the Invention
The present invention relates to an apparatus and a method for forming a deposited film on a substrate which is arranged in a vacuum vessel so as to oppose an discharging power application electrode arranged in said vacuum vessel by generating a plasma between said substrate and said electrode to decompose a raw material gas present in said vacuum vessel whereby depositing a film on said substrate.
2. Related Background Art
As a typical example of an electronic device in which a photovoltaic element is used, there can be mentioned a solar cell which converts sunlight energy or other light energy into electric energy. Recently, public attention has focused on the power generation system using a solar cell since the solar cell has advantages such that it is safe, can be readily handled and that it can be used as a power generation source of providing clean energy without causing CO2 buildup.
Separately, there have proposed a variety of amorphous semiconductor materials capable of being used in the production of an electronic device including a solar cell. Of these amorphous semiconductor materials, attention has riveted to amorphous silicon (a-Si) semiconductor materials since they have advantages such that they can be readily formed in a thin film having a large area with a relatively large design freedom with respect to their composition, they can be readily controlled in a wide range with respect to their electric and optical characteristics, and therefore, they are suitable as constituents of various electronic devices including solar cells. Particularly, a film comprising such a-Si semiconductor material (hereinafter referred to as xe2x80x9ca-Si filmxe2x80x9d) is more advantageous in comparison with a film comprising a crystalline silicon semiconductor material (hereinafter referred to as xe2x80x9ccrystalline Si filmxe2x80x9d). That is, the a-Si film has an absorption coefficient against an energy near the energy distribution peak of sunlight which is greater than that of the crystalline Si film, the a-Si film can be formed at a lower film-forming temperature (substrate temperature) than that in the formation of the crystalline Si film, and the a-Si film can be directly formed on a given substrate from a silicon-containing raw material by way glow discharge. In view of this, the a-Si film has been generally recognized as being suitable as a constituent of a solar cell and in fact, it has been widely using in the production of a solar cell.
Now, for a solar cell which has been considered to be important as a part of the future energy measure, it is an immediate necessity in terms of the research and development to lower the production cost and to more improve the performance. In order to attain the production of a solar cell at a desirably low production cost, attention has riveted to an amorphous silicon (a-Si) semiconductor material capable of readily formed in a thin film state. Hitherto, there have been proposed various a-si semiconductor films having a relatively high photoelectric conversion efficiency in terms of the performance. However, these a-si semiconductor films are still insufficient in terms of lowering the production cost. As one of the reasons for this, there can be mentioned a subject in that their film-forming speed (their deposition rate) is slow. For instance, in the case of producing a p-i-n junction type solar cell by means of a glow discharge decomposition method, its i-type semiconductor layer is generally formed at a deposition rate of 0.1 to 2 xc3x85/sec which is relatively slow. In this case, in order to complete the formation of the i-type semiconductor layer having a thickness of 4000 xc3x85, it takes about 30 minutes to about 2 hours which is relatively a long period of time. As a method of forming such a-Si semiconductor layer having a relatively large thickness at a high deposition rate, there has been an attempt in that 100% SiH4 gas or 100% Si2H6 gas is used. Besides, Japanese Patent Publication No. 56850/1993 describes that the deposition rate of a deposited film can be increased by shortening the distance between a power application electrode and a substrate capable of being served as an electrode.
Separately, Japanese Unexamined Patent Publication No. 232434/1994 discloses a process for mass-producing a photovoltaic element (a solar cell) using a roll-to-roll type film-forming apparatus. The roll-to-roll type film forming apparatus comprises a plurality of film-forming vacuum chambers having a discharge space communicated with each other through a gas gate which is provided between each adjacent film-forming vacuum chambers to isolate the inner atmosphere of the discharge space of one film-forming vacuum chamber from that of the other film-forming vacuum chamber, each of said plurality of film-forming vacuum chambers having a discharging power application electrode extending from a power source, a raw material gas introduction means extending from a raw material gas supply system and an exhaustion means connected to a vacuum pump, wherein a web substrate is moved to sequentially pass through the respective film-forming vacuum chambers and the respective gas gates while forming a functional deposited film comprising an amorphous silicon material or the like on said web substrate by each of the film-forming vacuum chambers, and said web substrate having a plurality of functional deposited films sequentially formed thereon is taken up by a take-up means while being wound thereon. The process using this roll-to-roll type film-forming apparatus excels in the productivity of a photovoltaic element (a solar cell).
Now, in the case where an amorphous silicon deposited film is formed on a substrate arranged in a deposition chamber by means of plasma CVD, in the vicinity of a region of the deposition chamber where a raw material gas is supplied into the deposition chamber, a plasma generated there contains non-decomposed raw material gas in a relatively large amount and because of this, there is a tendency in that a deposited film formed on said substrate in the vicinity of said region of the deposition chamber becomes to have an uneven film thickness or an inferior film property. In addition, also in the vicinity of a region of the deposition chamber where the deposition chamber is exhausted, a plasma generated there is liable to disorder, resulting in somewhat deteriorating a deposited film formed on said substrate. Particularly in the case of using such roll-to-roll type film-forming apparatus as above described, deposited films formed on a web substrate respectively in the vicinity of a region of each film-forming vacuum chamber where a raw material gas is supplied into the film-forming vacuum chamber (the discharge space) through the raw material gas introduction means and in the vicinity of a region of each film-forming vacuum chamber where the film-forming vacuum chamber is exhausted through the exhaustion means are also liable to have such defects as above described, where such defective deposited films result in forming an n/i interface and an i/p interface which greatly influence to characteristics of a photovoltaic element (a solar cell) produced.
In order to prevent such defective deposited films from being formed on the web substrate, there is known a manner in that an opening adjusting member as shown in FIG. 9 is provided in each film-forming vacuum chamber so as to cover the foregoing region of the film-forming vacuum chamber where such defective deposited film is formed on the web substrate.
However, in the case where the deposition rate of a deposited film formed a substrate (or a web substrate) is intended to increase by shortening the distance between the power application electrode and the substrate as previously described, there will be an occasion in that the distance between the power application electrode and the opening adjusting member becomes to be smaller than the distance (b) between the power application electrode and the substrate (see, FIG. 9).
In this case, disadvantages are liable to occur such that turbulence such as stagnancy or accumulation occurs in the flow of a raw material gas around a portion where the distance between the power application electrode and the substrate is small, whereby deposition of a thin film is hindered or/and dust including polysilane power is generated.
Besides, because the opening adjusting member also functions as a grounding electrode, in the case where the distance between the power application electrode and the opening adjusting member is diminished, the distance between the power application electrode and the substrate eventually comes to a result that it is diminished to the corresponding extend, where there is an occasion in that the deposition rate of the deposited film formed the substrate is locally increased.
In view of the foregoing disadvantages in the prior art, the present invention makes it an object to provide an improvement in a film-forming apparatus by means of plasma CVD comprising a substantially enclosed vacuum chamber whose inside being capable of being evacuated, a power application electrode for introducing a discharging power into said vacuum chamber, said power application electrode being arranged so as to oppose a substrate positioned in said vacuum chamber, and an opening adjusting member for adjusting a film-forming area of said substrate positioned in said vacuum chamber, so that a high quality deposited film can be stably and continuously formed on said substrate even when the distance between said power application electrode and said substrate is shortened in order to increase a deposition rate for said deposited film formed on said substrate.
Another object of the present invention is to provide a film-forming method using said plasma CVD film-forming apparatus.
A further object of the present invention is to provide a film-forming apparatus comprising at least a substantially enclosed vacuum chamber whose inside being capable of being evacuated, a power application electrode for introducing a discharging power into said vacuum chamber, a raw material gas introduction portion through which a raw material gas is introduced into said vacuum chamber, and an exhaustion portion through which said vacuum chamber is exhausted, said power application electrode being arranged so as to oppose a substrate positioned in said vacuum chamber, wherein a discharging power is introduced through said power application electrode to generate a plasma in a discharge space between said power application electrode and said substrate to decompose a raw material gas introduced through said raw material gas introduction portion whereby causing deposition of a film on said substrate, characterized in that at least said raw material gas introduction portion or said exhaustion portion is provided with an opening adjusting member for intercepting said plasma, and said opening adjusting member is provided to have a distance between said power application electrode, which is greater than a shortest distance between said power application electrode and said substrate.
A further object of the present invention is to provide a film-forming method using aforesaid film-forming apparatus in that said raw material gas which is flown from said raw material gas introduction portion toward said exhaustion portion is decomposed by said plasma generated in said discharge space between said power application electrode and said substrate to cause deposition of a film on said substrate.